help with categories

I want to write a function where I can pass in a parent category’s slug or id, which then checks whether or not the current post is in any of the parent category’s child categories, and if it is, it should return a list of all the child categories that the post is in.

I tried the ‘in_category’ and ‘post_is_in_descendant_category’ functions, but they only return true or false whether or not the post is in any child categories, they dont tell me specifically WHICH child categories the post is in.